The present invention relates to an improved process for the low pressure polymerization of olefins.
For the low pressure polymerization of olefins, it is known to use catalyst systems comprising a transition metal compound and an organometallic compound.
It is also known to attach the transition metal compound to oxygen-containing supports which have large pore volume, such as aluminas as described in Belgian Pat. No. 768,271 and halogen-containing aluminas as described in Belgian Pat. No. 773,227.
The catalyst systems thus produced are very active when compared with those wherein the transition metal compound is used directly. They lead to polymers which are characterized by a wide distribution of molecular weights and which are free from long chain branches, which renders them particularly suitable for use in extrusion-blowing.
However, these catalyst systems of the prior art possess a great disadvantage. In effect, they are most generally used in olefin polymerization processes carried out in the presence of a molecular weight regulator which is almost exclusively hydrogen. It has been recently found that these systems also catalyze the hydrogenation of olefins. A significant part of the olefin to be polymerized is thus converted into the corresponding saturated hydrocarbon, which is detrimental to the economics of the process. Furthermore, these catalyst systems produce polymers of very high average molecular weight. In order to decrease this average molecular weight, it is consequently necessary to increase the hydrogen concentration in the polymerization medium and/or to carry out the polymerization at a higher temperature. The unfortunate consequence of both these measures is to further increase the degree of hydrogenation of the olefin being polymerized.
Applicant has now discovered that the use of a new improved type of catalyst complex makes it possible to avoid the disadvantages connected with the use of the catalyst systems of the prior art, and in particular hydrogenation, while making it possible to produce, with an activity which is further improved, olefin polymers with a wide distribution of molecular weight of which is controlled more easily.